In known systems, vaporized fuel in the tank is stored in an active-charcoal filter which is connected via a closable tank-venting valve to the intake pipe of the internal combustion engine. When the tank-venting valve is open, air is drawn in by suction via a connection of the active-charcoal filter to the ambient air. This drawn-in air entrains the intermediately stored fuel and is conducted to the combustion. The vapor quantity drawn by suction is so controlled via the tank-venting valve that, on the one hand, the charcoal filter is adequately scavenged with air and, on the other hand, no intolerably large disturbances of the fuel/air ratio of the mixture supplied to the engine occur.
In this way, emissions of fuel vapor are intended to be avoided for reasons of environmental protection.
This task however is only satisfied by tank-venting systems which have no leaks. Statutory requirements with respect to diagnosis exist according to which even very small leaks (&lt;1 mm) can be detected during operation of the motor vehicle utilizing on-board means.
U.S. Pat. No. 5,398,661 discloses a method and an arrangement for checking the operability of a tank-venting system wherein a shutoff valve is mounted in the connection of the active-charcoal filter to the ambient. The diagnosis uses the underpressure in the intake pipe. For diagnosis, the tank-venting valve is first opened while the shutoff valve is closed and a check is made as to whether a pregiven minimum underpressure builds up within a predetermined time span. Thereafter, the tank-venting valve is again closed. The gradient of the pressure decay, which occurs in the interior of the tank-venting system, and of the subsequent pressure buildup is used to evaluate the tightness of the system. A rapid decay of the previously generated underpressure indicates that leakage is present. This underpressure method is preferably carried out during idle of the engine with the motor vehicle at standstill to obtain reliable diagnostic data. U.S. Pat. No. 5,383,437 discloses a further diagnostic method. This method operates with an additional overpressure pump which draws in a defined ambient air volume and compresses the same and conducts the same into the tank. The time in which the resulting pressure increase decays within the tank is used as a criterion for the tightness of the system.